The present application relates to passive infrared motion detection sensors and in particular, relates to a sensor which has improved features with respect to false alarms caused by small pets.
Passive infrared detectors focus radiation from an area to be monitored in a particular manner such that movement of a human intruder through the monitored space is detected. A Fresnel focussing arrangement (lens or mirror) focuses infrared radiation emitted by a human or pet target onto a passive infrared detector. To improve the response characteristics of the sensor, the Fresnel lens has multiple lensets and each lenset includes a focussing element defining an infrared beam that collectively covers the protected area. These beams increase in size as an increasing function of proportional to the distance from the detector. This characteristic of the Fresnel lens makes it difficult to distinguish between small pets located in a region close to the detector from a human target located at a substantial distance from the detector. In the closer region to the detector, the beams are quite small, and as such, a small pet will produce a signal similar in level to a person a substantial distance away from the detector.
As can be appreciated, small pets and in particular, cats, have substantial vertical mobility and are not confined to an area adjacent the floor. Some systems have attempted to design a dead or reduced bottom zone in the region close to the sensor in order to reduce problems associated with false alarms, caused by pets. The substantial vertical mobility of cats defeats this type of system.
U.S. Pat. No. 4,849,635 discloses a single passive infrared detector sensor where substantial gaps are provided between the sensing beams or zones. These zones are spaced such that a small pet must enter a dead zone as they move across the space. In contrast, a human target is much larger and taller and will therefore, produce a signal regardless whether he is standing at a position which at floor lever, is in a dead zone. With this arrangement, a pet produces a pulse signal with a very low component when the pet is in the dead zone, whereas a human target, although producing a pulsed signal, the signal is much more constant and can be easily distinguished from a pet. Unfortunately, with this system, a cat located at a high point in close proximity to the sensor will bridge two active zones much in the manner of a human.
It has also been known to use two different types of sensors to help distinguish between a human intruder and a pet. In particular, a microwave sensor in combination with a passive infrared sensor has been used. This type of dual technology sensor greatly increases the cost of the system and as such, is not particularly desirable. It has also been proposed to use a two element passive infrared detector and analyze the signal from the two detectors to distinguish between an intruder and a pet. Typically one element receives low radiation and one detector receives high radiation. S small pet does not have the height to trigger both detectors. This system again experiences some difficulties and also has the additional cost of the two element detection.
A better approach for distinguishing between small pets and human intruders is needed.